This invention relates to the electroless deposition of metallic silver on various substrates. In particular the invention relates to a novel reducing agent for the deposition of silver onto a substrate such as glass, plastic, ceramic or lacquer surfaces in addition to the coating of mirrors, decorative objects, and other non-conductive surfaces requiring a reflective, conductive or decorative metallic film.
The use of reducing agents for the electroless deposition of silver is well-known. Some of the earliest known reducing agents were agents such as formaldehyde, glucose and invert sugar. However, such prior art reducing agents tended to be unstable in use, often evolving hydrogen or decomposing to form sludge or other by-products. Dextrose, fructose, and arabinose are also known as prior art reducing agents.
U.S. Pat. No. 3,776,740 issued to Sivertz et al. disclosed the use of an aldonic acid (such as gluconic acid) and the salts thereof, (such as sodium gluconate) as improved reducing agents. Such reducing agents are stable in strong alkali solutions which permitted the formulation of nonexplosive silvering solutions. Their stability prevented the prior art problems of decomposition of the reducing agent in a highly alkaline solution.
U.S. Pat. No. 4,102,702 issued to the present inventor disclosed the use of a reducer containing a polyhydric alcohol which improved the efficiency of the silver deposition process. The preferred alcohol was sorbitol. U.S. Pat. No. 4,192,686 issued to Soltys disclosed the use of sorbitol in a nonexplosive two-part silver composition and process.
Reducing agents such as are disclosed in U.S. Pat. Nos. 3,776,740, 4,102,702 and 4,192,686 are extremely efficient when used at room temperatures. At higher temperatures (100.degree.-125.degree. F., 38.degree.-52.degree. C.) there is an increased possibility that such "cold reducers" will produce "reducer burn" (also referred to as "silver blush") wherein the silver film loses most of its adhesion to the glass surface. Such higher temperatures can be reached inadvertently in warmer climates.
Furthermore, the reducing agents disclosed in U.S. Pat. Nos. 3,776,740 and 4,102,702 in many cases produce a silver film which has a streaky blue-white coloration on the first surface. The "first" surface is the surface of the silver deposit farthest removed from the silver/glass interface. The streaks are caused by the rapid reduction of the silver when the reducer is used in a highly alkaline silvering solution. The streaks and blue-white coloration are also accentuated at higher temperatures.
As a result, the reducing agents such as sodium gluconate and polyhydric alcohols disclosed in U.S. Pat. Nos. 3,776,740 and 4,102,702 are not suitable for use where inadvertently high temperatures may be found or in applications where the appearance of the first surface is a primary concern. Such applications include decorative items, mirror frames, bottle cap closures and other reflective, conductive, and decorative applications.
Other known reducing agents, such as invert sugar, require higher temperatures to develop an efficient deposit of silver, e.g. temperatures in the range of 110.degree.-130.degree. F. (43.degree.-54.degree. C.). Below this range, they are very inefficient in depositing silver and thus are more costly to use.
The reducing agents of the present invention are stable in strong alkaline solutions permitting the use of nonexplosive silvering methods and formulations. They are more resistant to reducer burn (silver blush), than the gluconate and polyhydric alcohol reducers of the prior art, particularly at higher temperatures, and they operate efficiently within a temperature range of 70.degree.-130.degree. F. (21.degree.-54.degree. C.) which is broader than that of the prior art.
As a result, they produce a smoother, brighter and more uniform silver coating, without streaks, over a wider range of temperatures than previously known reducing agents. The reducers of this invention have been found to deposit silver not only on glass, but also on plastic surfaces, such as polycarbonate, poly-methylmethacrylate, and styrene. Thus they are suitable not only for mirrors, thermos bottles, Christmas ornaments and electroforming, but also on surfaces where a bright, highly reflective first surface is required such as on plastic bottle cap closures and decorative applications, etc.